Just a follow up, it turned out the leads on my DMM led me down a false path for a little while, the next time I took readings on the LM324 all the op-amps on it read the same as the other LM324 on the circuit board. It turned out to be a faulty LED (SCR TRIG), replaced it and now the treadmill is working once again.
The customer service No. was a dead end, should be called the customer no-service no., brouught to us by Nordic Trak.
Jeff, thanks for the helpful links; I also ran across an electronics surplus place that had several working boards in stock for 29.95. I was gonna post a link to the (hand-drawn) schematic once I'd gained permission from its author, still working on that.